In a subset of patients, ALS is associated with frontotemporal dysfunction (ALS-FTD). Clinically, ALS-FTD may present with a variable spectrum of cognitive and behavioural deficits. We report a 53-year-old ALS-FTD patient developing inappropriate sexual behaviour (ISB) manifesting as a vigorous sexual drive with exhausting demands for sexual intercourse and physical aggressions against his wife. This distressing symptom could be alleviated with the selective serotonin reuptake inhibitor sertraline. ISB is an embarrassing symptom for most patients and their caregivers and may therefore be under-reported. Since effective treatment is available, we suggest an open and proactive discussion of sexual issues in this patient group.